What I would do first is to disconnect the power stage fuses S1, S2, S101, S102 and 26V rails - S5,6,7,8.
C23,24,123,124 as Floobydust already mentioned can easily go wrong and need to be checked.
Check the voltage on the +-20VL, +-20VR the ground should be in the middle. if not, check REG4 as it seems to keep the GND in place from -20VL rail. If not, start removing the input resistors on REG1,2,3,5,6 and checking the voltages.
If symmetry and output voltages on REGs 1-6 are correct then you can try inserting S5,6,7,8 and check the 26v rails symmetry.
That's the algorithm I would follow.
wbr